Sonic data logging system

ABSTRACT

A sonic logging method involves the pre-processing of a detection signal by a downhole processing device to determine the arrival time of a sonic signal at a receiver which is then transmitted to the surface. A suitable sonic logging downhole tool includes a control device for controlling generation and reception of the sonic signal, an A-D converter for digitizing the detected signal at a predetermined sampling interval, a first memory for storing the digitized waveform, a second memory for storing a program for processing the digitized waveform, and a microprocessor capable of executing the stored program so as to determine an arrival time of the sonic signal arriving at the receiver.

DETAILED DESCRIPTION OF THE INVENTION

1. Field of the Invention

The present invention relates to logging technology for measuringphysical properties of underground formations, and, in particular, to alogging method and system using sonic waves.

2. Background

In order to explore underground resources, such as those providing oiland natural gas, use has conventionally been made of logging technologyby drilling a borehole into the ground, locating a measuring devicecalled a downhole tool or sonde inside the borehole so that is can bemoved up and down, and processing a measured signal from the downholetool with processing apparatus located at the surface and connected tothe downhole tool through a logging cable. In addition, sonic logginginvolving determining the speed of sound propagating through theunderground formation using a sonic wave generator and a receiverprovided on the downhole tool is also well known. For example, referenceshould be made to Jay Tittaman, “Geophysical Well Logging”, AcademicPress, Inc., and “Illustration Physical Exploration”, 1989, PhysicalExploration Society.

In conventional sonic logging a sonic wave in the form of a pulse isoutput by a sonic generator and transmitted into the ground, and thesonic wave propagating through the ground is detected by a receiver and,as an analog waveform, transmitted through a logging cable to the groundsurface processing apparatus which processes the analog waveform todetermine the arrival time of the sonic wave at the receiver. However,because of the unreliability of analog data received after transmissionalong a lengthy logging cable, a proposal has recently been made toconvert the analog signal into a digital signal at the downhole tool,and then to transmit this digital signal to the ground surfaceprocessing apparatus for the required processing (see, for example, A.R. Harrison, C. J. Randal, J. B. Aron, C. F. Morris, A. H. Wingnall, R.A. Dwoorak, L. L. Rulledge, and J. L. Perkins, “Acquisition and Analysisof Sonic Waveforms From a Borehole Monopole and Dipole Source for theDetermination of Compression and Shear Speeds and Their Relation to RockMechanical Properties and Surface Seismic Data”, SPE 20557, 1990,September 23-26, New Orleans, SPE 65th Annual Technical Conference andExhibition). However, in the above-identified literature (SPE 20557), asshown in its FIGS. 3 and 4, the whole of the digital signal exceeding apre-set threshold value is transmitted to the ground surface processingapparatus for analysis thereby. This requires the transmission of anexorbitant amount of digital data to the surface, and a broad bandwidthis required for the telemetry. In addition, since even that data whichis not necessarily required for sonic logging analysis is alsotransmitted, the efficiently of the operation is rather poor. And, sincea large amount of digital data is transmitted through a lengthy loggingcable, there is also a chance of errors in transmission.

Tasks to be solved by the Invention

The present invention, made in view of the points raised above, suggestsa sonic logging method and system capable of obviating the drawbacks ofthe prior art as described above. To achieve this it proposes ways ofminimising the amount of digital data to be transmitted to the groundsurface processing apparatus, reducing the bandwidth necessary for thetelemetry, and reducing also the possibility of data errors, therebyenhancing the system's reliability.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, therefore, thereis provided a sonic logging method for determining characteristics ofthe formations through which a borehole passes, which method uses adownhole tool which is moveably locatable up and down inside theborehole and which is coupled to ground surface processing apparatusthrough a logging cable, which tool is provided with at least one sonicwave generator and at least one receiver spaced apart from each otherand also with a downhole processing device operative coupled to saidgenerator and said receiver and also to said ground surface processingapparatus through said logging cable, in which method the downhole toolis first suitably located in the borehole, and then the sonic wavegenerator is caused to generate, and subsequently, receive a sonic wave,

which method is characterised by including the steps of:

(1) processing a detection signal from the receiver by the downholeprocessing device thereby to determine an arrival time of the sonic waveat its receiver; and

(2) transmitting the arrival time thus determined to the ground surfaceprocessing apparatus through the logging cable.

In accordance with another aspect of the present invention, there isprovided a sonic logging downhole tool, for use in a boreholecharacteristic determination method, which tool includes:

at least one sonic wave generator; and

at leas tone receiver capable of receiving the sonic wave after the wavehas travelled through a borehole ground formation or casing;

which tool is characterised by including a control device forcontrolling the generation and reception of the sonic wave, the controldevice comprising:

an analog-to-digital converter for digitising a detection signal fromthe receiver at a predetermined sampling interval;

a first memory for storing a waveform thus digitised;

a second memory for storing a predetermined program for processing thethus-stored digitised waveform; and

a microprocessor capable of executing the program stored in the secondmemory, thereby in operation processing the digitised waveform stored inthe first memory to determine an arrival time of a sonic wave generatedfrom the generator and arriving at the receiver.

In accordance with a further aspect of the present invention, there isprovided a sonic logging system which comprises the centralisation ofground surface processing apparatus and a sonic logging downhole tool ofthe invention as just defined.

In sonic logging, use is commonly made of a sonic waveform having acentral frequency of 15 kHz and a wavelength of 2.5 m. On digitisationthis, in the case of a 16 bit resolution, leads to 250×16 bits—4kilobits. In conventional digital sonic logging this amount of digitaldata is transmitted to a ground surface processing apparatus through alogging cable by way of telemetry communication, and the processing todetermine the arrival time and amplitude of the P (compression) wave iscarried out by the ground surface processing apparatus. However, what isactually required in sonic logging is basically only the arrival timeand amplitude of the P wave—rarely if ever is the remaining digitalwaveform data needed. Thus, the data which is actually necessary—the twotimes—comprises on digitisation a mere 2×16 bits=32 bits. So, as may beunderstood from this simple example, if the processing of the digitisedwaveform is carried out in the downhole tool itself, and only theresulting arrival time and amplitude are transmitted to the groundsurface processing apparatus, the amount of data to be transmittedthrough the logging cable can be reduced to one hundredth or less, andas a result the transmission efficiency is significantly improved, andthe occurrence of error is also significantly reduced. In addition, thelogging cable has an increased idle time, so that the logging cable canbe used for some other purpose. The present invention has been madeprincipally in view of these points.

Incidentally, although in accordance with the present invention, theprimary object is to determine downhole the sonic signal arrival timeand/or amplitude and then transmitting the result to the surface, it isof course also possible in the present invention—and in certain cases itis actually preferred—to transmit not all by only a selected portion ofthe digital signal up to the ground surface processing apparatus. Asexplained in more detail hereinafter, such a selective transmission ofdigital signal has the advantage of allowing the confirmation of thedata's reliability (by carrying out reprocessing with the ground surfaceprocessing apparatus).

An embodiment of the invention is now described, though by way ofillustration only, with reference to the accompanying Drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A schematic illustration showing a sonic logging systemconstructed on the basis of one embodiment of the present invention.

FIG. 2 (1) and (2) are schematic illustrations showing the arrangementof sonic transmitter and receivers in a sonic logging downhole tool ofthe present invention.

FIG. 3 A schematic block diagram showing one embodiment of a downholeprocessing unit provided in the sonic logging downhole tool of thepresent invention.

FIG. 4 A flow chart showing a detection routine based on one embodimentof a sonic logging method of the present invention.

FIG. 5 (a) and (b) are illustrations showing the digital waveforms foropen and casing type boreholes, respectively.

FIG. 6 (a) and (b) are illustrations showing a threshold detectionmethod in analog and digital sonic logging, respectively.

FIG. 7 (a) and (b) are illustrations showing the D.C. offset measuringtime period and the noise measuring time period, respectively.

FIG. 8 An illustration showing the arrival time detection time periodstart time and the peak amplitude measuring time period.

FIG. 9 An illustration showing the relation between the low and highthreshold detection and the peak amplitude detection.

FIG. 10 (a), (b) and (c) are illustrations showing the process forcarrying out the high threshold detection utilizing interpolationprocessing.

FIG. 11 An illustration showing how the peak amplitude is determined bythe quadratic interpolation using a parabolic line.

EXPLANATION OF NUMERALS

1: Downhole Tool

2: Logging Cable

3: Ground Surface Processing Apparatus

4: Borehole

8: Downhole Processing Unit

12 c: ADC Converter

13 a: Digital Signal Processor

13 c: Telemetry Interface

13 d: Program Memory

Tt: Arrival Time

T₀: Sonic Wave Firing Time

Td; Detection Start Time

Tp: Previous Arrival Time

T_(HT): High Threshold Arrival Time

T_(LT): Low Threshold Arrival Time

E1: First Peak

E2: Second Peak

MODES EMBODYING THE INVENTION

FIG. 1 is a schematic illustration showing a sonic logging systemconstructed in accordance with one embodiment of the present invention.As shown in FIG. 1, the present sonic logging system includes a downholetool (also called sonde) 1 movable up and down within a borehole 4drilled into the ground. The downhole tool 1 is elongated in shape andon an outer peripheral surface of its housing are mounted a sonictransmitter T and a pair of receivers R1 and R2 spaced apart from eachother over a predetermined distance in a vertical direction and alsospaced apart from the transmitter T over a predetermined distance in thevertical direction. Inside the housing of downhole tool 1 is sealinglyprovided a downhole processing unit 8. An example of a specificstructure of downhole processing unit 8 will be described in detaillater with reference to FIG. 3. The downhole processing unit 8 isconnected to the sonic transmitter T and receivers R1 and R2 throughinternal interconnections (not shown) and a sonic wave in the form of apulse is emitted into the underground formation from the sonictransmitter T in accordance with an instruction from the downholeprocessing unit 8. The sonic wave propagates through the undergroundformation along the borehole 4 in the vertical direction and is receivedby receivers R1 and R2, respectively, so that respective detectionsignals from the receivers R1 and R2 are supplied to the downholeprocessing unit 8. In this manner, if respective arrival times T1 and T2of the sonic wave at respective receivers R1 and R2 after having beenemitted from the sonic transmitter T and propagated through theunderground formation are detected and a difference between thesearrival times, i.e., ΔT=T1−T2, is calculated, then the propagation timeof the sonic wave which has propagated through the underground formationover the vertical distance between the pair of receives R1 and R2 can becalculated. Accordingly, from this vertical distance and the propagationtime, the speed of the sonic wave which has propagated through theunderground formation can be calculated. And, since the propagation timeof a sonic wave is associated with the structure of undergroundformation, it is possible to analyze such a structure.

It is to be noted that the borehole 4 shown in FIG. 1 is a so-called“bare borehole” so that the wall of borehole 4 is formed by theunderground formation. In this case, as described above, since the sonicwave which has propagated through the underground formation is detected,it is possible to analyze mainly the structure of the undergroundformation. On the other hand, use may be made of a cased borehole inwhich case a cylindrical casing is fitted along the wall surface ofborehole 4. If a casing is present, since th sonic wave propagatesthrough the casing, the sonic wave which first arrives at a receiver isthe one which has propagated through the casing. In this case, bymeasuring the amplitude of first P wave, the cementing between thecasing and the underground formation, the connecting condition betweensegments of the casing or the like can be evaluated. In the soniclogging, in particular, since the downhole tool 1 is desired to belocated at the center of borehole 4, in the embodiment shown in FIG. 1,centering members 1 a, 1 a are provided at appropriate top and bottomlocations of downhole tool 1. It is to be noted that in the case ofmoving the downhole tool 1 up and down along the borehole 4, thecentering members 1 a are preferably set in their retracted positions.

The top portion of downhole tool 1 is connected to a logging cable 2 sothat the downhole tool 1 is suspended by the logging cable 2. Thelogging cable 2 not only holds the downhole tool 1 in a suspendedcondition mechanically, but also includes transmission lines (now shown)so as to allow to transmit signals electrically or optically to and fromthe downhole processing unit 8. The logging cable 2 is wound around adrum 6 located at a ground surface 5 and is connected to an extensionlogging cable 2′ at a center shaft of the drum 6. And, the extensionlogging cable 2′ is connected to a ground surface processing apparatus3, which is typically comprised of a computer system.

FIG. 2 illustrates a few examples of an arrangement of sonictransmitters and receivers of a downhole tool constructed in accordancewith another embodiment of the present invention. That is, in theembodiment shown in FIG. 1, provision is made of a single sonictransmitter T and a pair of receivers r1 and r2, whereas in a downholetool 1′ of FIG. 2(1), provision is made of a pair of sonic transmittersUT and LT and a pair of receivers R1 and R2. In this case, a sonic waveemitted from the upper sonic transmitter UT arrives at the pair ofreceivers R1 and R2 at arrival times TT1 and TT2, respectively, and, onthe other hand, a sonic wave emitted from the lower sonic transmitter LTarrives at the pair of receivers R1 and R2 at arrival times TT3 and TT4,respectively. On the other hand, in a downhole tool 1″ of FIG. 2(2), twopairs of receivers R1 and R3 and R2 and R4 arranged between a pair ofsonic transmitters UT and LT, in which a sonic wave from the upper sonictransmitter UT arrives at the pair of receivers R2 and R4 at arrivaltimes TT2 and TT1, respectively, whereas, a sonic wave from the lowersonic transmitter LT arrives at the other pair of receivers R1 and R3 atarrival times TT3 and TT4, respectively. In this manner, by providing astructure in which sonic waves from separate sonic transmitters aredetected at the same formation location, the reliability of data can beincreased and a detailed analysis of an underground formation can bemade.

FIG. 3 is a block diagram showing a detailed structure of an embodimentof the downhole processing unit 8 inside the downhole tool 1. In FIG. 3,the downhole processing unit 8 includes a bottom connector 10 which iselectrically connected to the sonic transmitter T and receivers R1 andR2 of the downhole tool 1. In the embodiment shown in FIG. 1, since thedownhole tool 1 has a pair of receivers R1 and R2, these are connectedto the bottom connector 10 and thus to respective correspondingpreamplifiers 11 a in a preamplifier section 11. Besides, the sonictransmitter T is also connected to the bottom connector 10 so that afire control signal for firing (emitting) a sonic wave by energizing thesonic transmitter T is supplied to the sonic transmitter T and a highvoltage for firing a sonic wave is supplied from a high voltage powersupply 14 b of a power supply section 14 to the sonic transmitter T. Inaddition, a detection signal of sonic wave firing time T₀ is suppliedfrom the sonic transmitter T. In the preamplifier section 11 is alsoprovided a signal level converter 11 b which is connected to respectivepreamplifiers 11 a. This signal level converter 11 b may be used as again adjustment unit in the case where the downhole tool 1 has twochannels.

The preamplifier section 11 is connected to a data acquisition section12 through a but, which has two parallel paths, each including amultiplexer 12 a, an amplifier (incorporating an anti-aliasing filter)12 b, and an analog-to-digital converter 12 c. And, the data acquisitionsection 12 is connected to a control section 13 through a bus. Thecontrol section 13 includes a digital signal processor (DSP) 13 a, adigital control interface 13 b, a telemetry interface 13 c, a programmemory 13 d and a data memory 13 e, which are connected by buses eachother with the DSP 13 a at the center. In this embodiment, although useis made of ADSP-2101 commercially available from Analog Devices, Inc.for the DSP 13 a, the present invention should not be limited only tothe use of such a specific processor and use may also be made of anyother general purpose microprocessors, micro-controllers or the like.The program memory 13 d may be constructed by one or more of commonnon-volatile memories, such as ROM, PROM, EPROM and EEPROM. Inparticular, it should be noted that a sonic logging routine (e.g.,Digital First Arrival Detection, or simply DFDA) for implementingvarious unique sonic logging functions of the present invention, whichwill be described in detail later, is stored in this program memory 13d. That is, detection signals from the receivers R1 and R2 are digitizedby the A/D converter 12 c and the resulting digital waveforms are storedin the data memory 13 e, so that it is possible for the DSP 13 a toprocess the digital waveforms stored in the data memory 13 e inaccordance with a sonic logging program stored in the program memory 13d to thereby determine the arrival time, amplitude or the like of thefirst P wave. It is also to be noted that the data memory 13 e iscomprised of a RAM for storing working data. The digital controlinterface 13 b is particularly in charge of timing and controls thetiming in operation of preamplifier section 11, data acquisition section12 and sonic transmitter T. In addition, it is to be noted that, as analternative embodiment of the present invention, the processor 13 a andprogram memory 13 d may be constructed in the form of firmware, such aslogic gates, in place of a microprocessor.

The control section 13 is connected to a top connector 15 through a busand the top connector 15 is connected to a telemetry cartridge (notshown) which, in turn, is connected to one end of the logging cable 2.Thus, the DSP 13 a can transmit or receive data to or from the groundsurface processing apparatus 3 by way of telemetry communication throughthe logging cable 2. In addition, a low voltage power supply 14 a isprovided in the power supply section 14 for supplying power to thepreamplifier section 11, data acquisition section 12 and control section13.

Now, referring to FIGS. 4 through 11, a unique downhole sonic loggingfunction and method of the present invention will be described in detailbelow. FIG. 4 illustrates a flow chart of the DFAD (Digital FirstArrival Detection) program for determining the arrival time and theamplitude at downhole, i.e., inside a borehole, among the sonic loggingprogram of the present invention stored in the program memory 13 d shownin FIG. 3. It is to be noted that the DFAD routine shown in FIG. 4 isonly a portion of the sonic logging of the present invention. That is, adownhole sonic logging sequence of the present invention includesbasically the following four routines.

(1) Sonic Transmitter Activation Routine

(2) Data Acquisition and Digitization Routine

(3) DFAD Routine

(4) Telemetry transmission of DFAD Results to Ground Surface ProcessingApparatus

That is, in accordance with the downhole sonic logging sequence of thepresent invention, in the first place, in routine (1), the DSP 13 asupplies a command for emitting a sonic wave to the sonic transmitter Taccording to the sonic logging program stored in the program memory 13d. Then, in routine (2), the DSP 13 a detects sonic fire time T₀ andsamples detection signals from the receivers R` and R2 at apredetermined sampling interval (e.g., 10 micro-seconds), and has themdigitized by the A/D converter 12 c. The resulting digital waveform isstored in to the data memory 13 e. It is to be noted that also in theseroutines (1) and (2), the DFAD routine is called as desired to use thedata stored in the DFAD. Then, it enters into the flow of DFAD routine(FIG. 4), in which the digital waveform stored in the data memory 13 eis processed to thereby determine the arrival time and/or amplituderegarding the first P wave in the digital waveform. And, then, it entersinto routine (4), where the arrival time and/or amplitude thusdetermined are set into a telemetry frame, which, in turn, istransmitted to the ground surface processing apparatus 3 through thetelemetry interface 13 c or set in a stand-by state until the telemetryframe becomes full. The above-described downhole sonic logging sequenceis repetitively carried out at different depths in the borehole 4 whilemoving the downhole tool 1 along the borehole 4 to thereby obtain a logalong the longitudinal direction of the borehole 4.

Now, with reference to the flow chart of FIG. 4, individual routines fordetermining the arrival time and the amplitude of the first P wave froma digital waveform in the DFAD routine will be described in detailbelow.

As described above, once a digital waveform, which has been obtained bysampling with a predetermined gain and a predetermined sampling interval(e.g., 10 micro-seconds) after firing of a sonic wave and digitizing thesampled data (e.g., each sample being a 16bit signed integer value), isstored into the data memory 13 e, the DSP 13 a, at step 20 of FIG. 4,calls the DFAD routine stored in the program memory 13 d.

Although not shown in the flow chart of FIG. 4, in the presentembodiment, at the start of the DFAD it is first determined whether adigital waveform is to be inverted or not depending on the kind ofborehole 4. That is, as described before, the borehole, i.e., open typewithout a casing, as shown in FIG. 1 or a cased borehole, i.e., cladtype with the wall surface of borehole 4 being clad with a cylindricalcasing. Thus, depending on whether the borehole 4 is either of these twotypes, the waveform pattern of a detected signal differs and thus itsprocessing also differs. In the case of an open type borehole, mainly,the arrival time of the first P wave which has propagated through theground formation along the borehole and has been detected by therespective receivers is determined and then based on the arrival timethus determined the propagation speed of a sonic wave, or its inverse ofslowness, is calculated. Such parameters as the propagation speed ofsonic wave and the slowness are related to the structure of anunderground formation, so that such an underground formation structure(e.g., existence of oil or the like) can be analyzed by mapping theseparameters along the longitudinal direction of a borehole. On the otherhand, in the case of a casing type borehole, mainly, the amplitude ofthe first P wave, which has propagated through the casing, is determinedand then based on the magnitude of the amplitude the bond strength (bondindex) or the degree of cementing between the casing and the groundformation is evaluated. That is, since the higher the bond strengthbetween the casing and its surrounding ground formation, the moreleakage of the sonic energy to the surrounding ground formation, theamplitude of the first P wave which is detected by the receiver Rbecomes smaller.

Now, as shown in FIG. 5(a), in the case of logging with an open typeborehole, since it is common to determine the arrival time T using thesecond peak E2 rather than the first peak E1 of P wave, the polarity ofthe detected digital waveform is inverted. It is to be noted thatalthough the digital waveform, in fact, has individual discrete datavalues spaced apart from one another at the sampling interval, it isshown as a smooth and continuous curve in FIG. 5 as a matter ofconvenience. As explained before, T₀ indicates the sonic wave firingtime. On the other hand, as shown in FIG. 5(b), in the case of loggingwith a casing type borehole, since it is common to determine amplitude Ausing the first peak E1, the polarity of the digital waveform in thiscase is not inverted. Since the second peak E2 is opposite in polarityto the first peak E1, when the present DFAD routine is to be used inboth of open type and casing type boreholes, it is necessary to invertthe polarity of the data of a digital waveform when determining thearrival time T using the second peak E2. And, in this manner, in thecase when processing is carried out with the DFAD routine by invertingthe polarity of the data of a digital waveform, it is necessary toreinvert the data of the digital waveform after the processing with theDFAD routine so as to return to the original condition. It is to benoted that, in the present embodiment, since it is so structured todetermine the arrival time T using the second peak E2 in logging with anopen type borehole, it is necessary to invert the polarity of a digitalwaveform; however, if it is so structured to determine the arrival timeT using the first peak E1 even in the case of an open type borehole,then it is, of course, not necessary to invert the polarity of a digitalwaveform.

Then, as shown by step 21 of FIG. 4, DC offset measurement is carriedout. This step is to establish the zero level of the base line of awaveform, i.e., the detection level under the condition in which nosonic wave has yet arrived at the detector R. In accordance with thepresent invention, a digital waveform is obtained by digitizing adetection signal by the A/D converter 12 c inside the downhole tool 1,but the base line of the digital waveform does not necessarily agreewith the zero level in the output of D/A converter 12 c precisely. Inparticular, in this embodiment, use is made of a 16bit ADC (or A/Dconverter) as the A/D converter 12 c, in which case such a difference isparticularly noticeable. Thus, in order to take into consideration adifference between the zero level in the output of A/D converter 12 cand th base line of the digital waveform, such a difference needs to bemeasured as a D.C. offset. As shown in FIG. 7(a), a D.C. offsetmeasuring time period T_(OFF) may be set at an arbitrary location of thebase line portion of a digital waveform, but it must be set prior to thestart of an arrival time detection period. In the example shown in FIG.7(a), the D.C. offset measuring time period T_(OFF) is set to be 100micro seconds and the sampling interval is set at 10 micro seconds, sothat there are shown eleven sampled data points. The D.C. offset isdetermined by calculating the average of these data points. It is to benoted that, as will be described later, the D.C. offset thus obtained isused for adjusting the location of the threshold to be used fordetermining arrival time Tt and also for correcting the measured valueof the peak amplitude.

Then, noise measurement is carried out at step 22 of FIG. 4. In thisnoise measurement, the amplitude of the maximum positive noise peak inthe base line of a digital waveform up to the start of the arrival timedetection time period is determined. The maximum noise amplitude thusdetermined is used for setting the noise level of the amplitude of thefirst P wave which is to be determined later and also for evaluating theamplitude of the P wave thus determined. Preferably, for example, withintermediate and high noise thresholds set, the maximum noise amplitudethus determined is classified by determining whether it is at a lownoise level, intermediate noise level or high noise level and then it isused for evaluating the result obtained by this routine at evaluationstep 30 which is the last step in the routine. In the embodiment shownin FIG. 7(b), the noise measurement is set such that a noise detectionperiod T_(NOISE) terminates at the start of the arrival time detectiontime period, and since there are eleven sampling points at the intervalof 10 microseconds, it is set at 100 micro seconds. And, in noisemeasurement, the maximum positive peak NLP in this noise detection timeperiod is determined. In this case, the noise peak is determined, forexample, by finding the maxim positive amplitude whose preceding andfollowing adjacent sampling points are both smaller in amplitude. Thenoise detection period may be set at an arbitrary time period at anarbitrary base line position depending on various conditions as long asit remains prior to the start of the arrival time detection time period.

Then, at step 23 of FIG. 4, an automatic gain control routine is carriedout. In the automatic gain control routine, as shown in FIG. 5(a), indetecting arrival time Tt of the first P wave, an intersecting point ofthe curve directed toward positive peak E2 of the first P wave with apredetermined threshold is determined and the arrival time is determinedby the time from the sonic wave firing time T₀ to the intersectionpoint. The reason why the arrival time is determined by using anintersecting point with a selected threshold rather than the base lineof a digital waveform, or the zero cross with the zero level, isbecause, as described before, the base line of a digital waveformincludes noise as well as D.C. offset, an error may occur if use is madeof the zero cross with the base line of a digital waveform. Thus, it isso structured that, in view of the detected noise and D.C. offset, athreshold level is determined and an intersecting point with thisthreshold is detected to determine arrival time Tt. Thus, such athreshold is determined as a predetermined ratio relative to theamplitude of peak E2 of first P wave. If the threshold level to be usedfor determining arrival time Tt is defined as a predetermined ratiorelative to peak E2 in this manner, in the case where the amplitudevalue of peak E2 of the next digital waveform differs from the amplitudevalue of peak E2 of the preceding digital waveform, it becomes necessaryto adjust the threshold level used for peak E2 of the preceding digitalwaveform when arrival time Tt is to be determined for the precedingdigital waveform. The automatic gain control, for this reason,determines a detection gain to be used for detection processing andadjusts the threshold level using the detection gain for each digitalwaveform, thereby securing the determination of arrival time Tt using athreshold level which is determined with a predetermined ratio relativeto peak E2 at all times.

What is described in the preceding paragraph will be described more indetail with reference to FIGS. 6(a) and (b). FIG. 6(a) illustrates thecondition in which arrival time Tt is determined using a threshold levelin the prior art analog sonic logging. In this case, when a sonic waveis detected by a downhole tool, its detection signal is transmitted to aground surface processing apparatus by way of telemetry communication sothat the determination of the arrival time is carried out by the groundsurface processing apparatus. Thus, the waveform shown in FIG. 6(a) is acontinuous analog waveform. Incidentally, Td is detection start time forthe current arrival time detection and Tp is the arrival time which hasbeen determined by the preceding arrival time detection processing. Anarrow directed from Tp toward Td indicates the fact that start time Tdof current detection time period is set earlier than the arrival timeTp, which has been determined by the preceding processing, by an amountof time, which has been previously selected or is determined by carryingout a predetermined process. Tt is the arrival time which has beendetermined by the current processing.

In FIG. 6(a), there is shown the case in which the gain control iscarried out to maintain the amplitude of peak E2 at 5,000 mV through thevariable gain control in an analog detection system. It is shown thattwo thresholds, i.e., high threshold of 1,000 mV and low threshold of250 mV, are set. Thus, even if peak E2 of the next waveform has anamplitude value which differs from that of the peak E2 of the precedingwaveform, the threshold level, which has been set once, does not vary,so that the detected arrival time and the amplitude value of peak E2differ in value from waveform to waveform.

On the other hand, FIG. 6(b) shows the detection principle based on oneembodiment of the present invention. Although FIG. 6(b) shows adetection signal by a continuous line for the matter of convenience, itis to be noted that this line, in fact, is a collection of discretesample points since it is a digital waveform. In FIG. 6(b), Ad is adesired peak amplitude of peak E2, HT is a high threshold and LT is alow threshold. It is to be noted that desired peak amplitude Ad is anamplitude for which the automatic gain control desires as a peak, andthe detection gain for the next detection is adjusted based on thecurrent detection gain, and a ratio between the preceding Ad and thepreceding amplitude. The remaining parameters have the same meaning asthose of FIG. 6(a). In accordance with the present invention, sinceprocessing is carried out within a downhole tool, it is impossible forthe operator to control the gain of the amplifier. In addition, at thedownhole processing unit 8, a digital waveform is sampled at apredetermined gain. Thus, in this case, if the threshold level is fixedat a constant level, the ratio of peak E2 to amplitude Ad of eachwaveform varies. Under the circumstances, in order to maintain thelevels of thresholds HT and LT relative to peak E2 at predeterminedratioes for each of waveforms, the detection gain is calculated for eachof the waveforms based on the amplitude of detected E2. And, then,threshold levels HT and LT are set for each of the waveforms based onthe detection gain thus calculated.

The reason why the detection gain is calculated in this manner isbecause, in processing a digital waveform, the arrival time and theamplitude value of peak E2 are determined using the detection gain whichhas been determined for the preceding waveform or which has beenselected by the user in the case of the very first processing, and,then, the detection gain is modified to the current waveform based onthese values. And, the current waveform is processed using the detectiongain thus modified to thereby determine refined arrival time andamplitude value of peak E2. By repeating such a process twice, thearrival time and the amplitude value of peak E2 can be determined basedon the detection gain of the current waveform. As described above, inthe above-described embodiment, since thresholds HT and LT aredetermined at predetermined ratioes relative to peak E2 for each of thewaveforms, it is necessary to determine the detection gain for each ofthe waveforms, and, for that purpose, a digital waveform is processed todetermine the detection gain of that waveform in the automatic gaincontrol routine. In a preferred embodiment, in the automatic gaincontrol routine, maximum and minimum detection gains are set dependingon the conditions, and it is determined to be a valid detection gainonly when the calculated detection gain falls between them.

Then, at step 24 of FIG. 4, using the detection gain as calculatedabove, various threshold levels, including high and low threshold levelsHT and LT, are calculated. For example, in one embodiment, (1) desiredamplitude (100%), (2) low threshold (15%), (3) high threshold (20%), (4)intermediate noise threshold (4%) and (5) high noise threshold (40%) arecalculated.

Then, at step 25 of FIG. 4, an arrival time detection time period fordetecting arrival time Tt in a digital waveform is set. The arrival timedetection time period is to determine to use which portion of a digitalwaveform for the detection of arrival time Tt. Such a detection timeperiod can be set variably or at a fixed value for each of waveforms. Inorder to carry out the detection processing of arrival time Ttexpeditiously and efficiently, it is desirable to set the detection timeperiod at a location where arrival time Tt is likely to be present,i.e., such that the detection time period starts immediately before thevicinity of an intersection point with a threshold. For example, thedetection time period for the current and next digital waveform is setbased on the processed result of the preceding digital waveform, and,since it is not likely that the arrival time of the current digitalwaveform varies from the arrival time of the preceding digital waveformsignificantly, it is preferable to determine the start of the detectiontime period by going back over a predetermined time period based on thearrival time of the preceding digital waveform. In particular, in thecase of an open type borehole, since the sonic wave which propagatesthrough the underground formation is detected, it is effective to setthe detection time period varyingly for each of the digital waveforms.On the other hand, in the case of a casing type borehole, since thearrival time remains the same for the size of a given casing, it ispossible to use a fixed detection time period.

In FIG. 8, Td is the start time of arrival time detection time periodand it is set by going back over a predetermined time period from thearrival time Tp which has been determined at the preceding processing,and it is shown that the arrival time Tt is set at a positive goingintersection point of a digital waveform with the high threshold HT forthe first time and the detection time period terminates there. Upondetermination of arrival time Tt, an amplitude measuring time period forpeak E2 starts at step 29 of FIG. 4 and the amplitude measuring timeperiod ends at a predetermined time Ta. In addition, FIG. 9 shows that adigital waveform is at first at output zero level A₀ of the A/Dconverter and it changes to the first positive peak E2. And, the pointin time when the digital waveform intersects with the low threshold LTfor the first time is indicated as T_(LT) and the point in time when thedigital waveform intersects with the high threshold HT for the firsttime is indicated as T_(HT). The time T_(HT) becomes the arrival time Ttdetermined for this waveform. In addition, the amplitude value of peakE2 is indicated by Ad.

Then, at steps 26 through 28 of FIG. 4, a low and high thresholddetection routine is carried out. This routine can be consideredfundamentally as divided into two stages. That is, (1) low and highthreshold detection using a sampled waveform data and (2) final highthreshold detection by interpolation of waveform data. In the firststage, as shown in FIG. 10(a), digital waveform sample points sampled atan interval of 10 micro seconds are scanned in a detection time periodset as described above to thereby find the first sample points whichhave intersected the low and high thresholds LT and HT for the firsttime in the positive direction, respectively. In the present case, it isthe sample point T_(LT) that has intersected the low threshold LT forthe first time in the positive direction and it is the sample pointT_(HT) that has intersected the high threshold HT for the first time inthe positive direction. Thus, according to the processing of the firststage, the sample point T_(HT) is tentatively determined as the arrivaltime Tt. This is because, in this embodiment, the arrival time isdefined as a point in time when the first P wave intersects with thehigh threshold HT for the first time in the positive direction. Thus, ifthe arrival time is defined differently, a specific method fordetermining the arrival time may differ. In this manner, although thesample point T_(HT) is tentatively determined as the arrival time ofthis digital waveform as a result of the first stage of this routine,there may be a case in which the sample point T_(HT) is, in fact,located far apart from the high threshold HT. This is because therespective sample points are spaced apart over the sampling interval of10 micro seconds at data acquisition. Thus, even if the samplinginterval is sufficiently fine or the sampling interval is relativelycoarse but the sample point T_(HT) which has been determined by thefirst stage intersects with the high threshold HT with a sufficientaccuracy, this routine can be terminated at the end of the first stage.

This routine allows to determine the arrival time at high accuracy byaccurately estimating an intersection point between the digital waveformand the high threshold HT by carrying out an interpolation process atthe second stage. This second stage corresponds to the loop includingstep 28 in FIG. 4. In the present embodiment, this interpolation processis carried out also in two stages. That is, in the first place, bandlimited interpolation shown in FIG. 10(b) is carried out and then linearinterpolation shown in FIG. 10(c) is carried out. As shown in FIG.10(b), the band limited interpolation is applied from the sample point(in this case, sample point T_(LT)) which is immediately preceding thesample point T_(HT), which has been tentatively determined as arrivaltime Tt, to thereby find interpolation points at an interval of 2.5micro seconds between the sample points at 10 micro seconds in thedigital waveform. In this example, since the sampling interval of adigital waveform is 10 micro seconds, three interpolation points arefound between the two adjacent sample points at 10 micro seconds. Then,processing to find a point (sample point or interpolation point) whichintersects the high threshold HT in the first place in the positivedirection is carried out for both the sample and interpolation points.In this example, interpolation point T_(HT) is found as a result of suchprocessing so that this interpolation point T_(HT)′ is tentativelydetermined as refined arrival time Tt. Then, as shown in FIG. 10(c), twopoints (in this example, sample point T_(LT) and interpolation pointT_(HT)′) which are closest to the intersection with the high thresholdHT are found, and linear interpolation is carried out for these twopoints to calculate an intersection point T_(HT)″ with the highthreshold HT, which is then determined as the final arrival time Tt. Asdescribed above, in the present embodiment, the interpolation processingis carried out in two stages, i.e., first stage with the band limitedinterpolation and the second stage with the linear interpolation, but itis, of course, also possible to provide a structure which carries outonly the band limited interpolation or linear interpolation depending onthe application conditions.

Now, the above-described band limited interpolation is well known as amethod for resampling or determining interpolation points between samplepoints at a higher resolution than the sample points which have beenobtained for the first time by digitization. In this respect, referenceshould, for example, be made to A. V. Oppenheim and R. W. Shafter,“Reconstruction of a Bandlimited Signal from its Samples”, Discrete TimeSignal Processing, Inglewood cliff, N.J., U.S.A., Prentice Hall, 1989.In the bank limited interpolation, use is made of sample points of awaveform themselves for reconstructing a waveform signal between thesample points. That is, interpolated amplitude y(t) at a given point ina waveform at time t can be calculated by the following band limitedinterpolation equation.${y(t)} = {\sum\limits_{i = 0}^{n}\quad {y_{i}\frac{\sin \left( {{\pi \left( {t - t_{i}} \right)}/T} \right)}{{\pi \left( {t - t_{i}} \right)}/T}}}$

where,

y_(i): amplitude of ith waveform sample,

t_(i): time of ith waveform sample,

T: sampling interval,

n: number of waveform samples.

However, the above-described band limited equation requires to use allof waveform samples and also to apply a filter same in length as thewaveform. In order to avoid the calculation of such an exorbitant amountof data, in a preferred embodiment of the present invention, use is madeof modified band limited interpolation in which a shorter filter lengthis used and a fewer number of waveform samples around a point to beinterpolated can be used. This modified band limited interpolationequation is expressed as follows:${y(t)} = {\sum\limits_{i = {i_{t} - \frac{f - 1}{2}}}^{i_{i} - \frac{f - 1}{2}}\quad {y_{i}\frac{\sin \left( {{\pi \left( {t - t_{i}} \right)}/T} \right)}{{\pi \left( {i - t_{i}} \right)}/T}}}$

where,

i_(t): time index closest to t,

f: filter length in sample number, odd number,

Thus, in the preferred embodiment of the present invention, use is madeof this modified band limited interpolation equation to relax theconstraints in calculation and increase the processing speed.

Then, at step 29 of FIG. 4, an amplitude measuring process is carriedout. In the amplitude measuring process, amplitude Ap of the firstarrived peak is measured. This amplitude measurement is carried outafter the detection of the high threshold. The amplitude thus measuredis used in the automatic gain control routine, which determines the highand low thresholds HT and LT as described before. In the preferredembodiment of the present invention, this peak amplitude is determinedfrom waveform interpolation points interpolated at a sampling intervalwhich is finer than the sampling interval at the time of dataacquisition. Besides, in particular, in the case of a casing typeborehole, in determining the final amplitude, use is made of a quadraticinterpolation equation which is approximated by a parabolic line.

A peak amplitude measuring time period is set prior to the detection ofarrival time when the arrival time detection time period is set. Thepeak amplitude measuring time period is from Td to Ta in FIG. 8. Themeasurement of peak amplitude can be carried out independently of or atthe same time with the detection of low and high thresholds, so that,even if the detection of low and high thresholds is not carried out, thedetection of peak amplitude can be carried out. On the other hand, inthe case where the high threshold is detected at time Tt, time Ta isvaried such that the peak amplitude measuring time period terminatesafter a predetermined time period after time Tt. The digital waveformsamples within the peak amplitude measuring time period are scanned tofind the largest peak. If the amplitudes of both of the adjacentpreceding and following sample points (left and right in the illustratedexample) are smaller than the amplitude of a current sample point, thecurrent sample point is extracted as a peak and the peak having thelargest amplitude among the peaks thus extracted becomes peak amplitudeAp. The amplitude measurement is adjusted for D.C. offset A₀. It isimportant to set the termination time Ta such that, if an intersectionwith the high threshold has been detected, any following P waves whichfollows the first P wave are not detected. If an intersection with thehigh threshold has been detected, the scanning of the sampled datapoints can be stopped and scanning of interpolation points between thesampled data points can be initiated.

That is, in accordance with the preferred embodiment of the presentinvention for carrying out amplitude measurement, after finding theinterpolation point T_(HT)′ by carrying out the high threshold detectionusing the digital waveform samples shown in FIG. 10(a) and the highthreshold detection by the band limited interpolation shown in FIG.10(b), the time after 25 micro seconds from that interpolation point isset as the termination point Ta of the peak amplitude measuring timeperiod. And, among the sample points and interpolation points at aninterval of 2.5 micro seconds in the peak amplitude measuring timeperiod, the point having the largest amplitude is determined as peakamplitude Ap. If the peak amplitude detection is to be carried outfollowing the detection of high and low thresholds, it is of coursepossible to carry out the peak amplitude detection processing using thedata of FIG. 10(b). In particular, in the case of a casing typeborehole, as shown in FIG. 11, more refined final peak amplitude Ap canbe determined by carrying out quadratic interpolation with parabolicapproximation using the three points near the peak. Since the amplitudemeasurement without the quadratic interpolation is normally sufficientfor the automatic gain control routine, the determination of peakamplitude using the quadratic interpolation is carried out only in thecase of a casing type borehole. However, if precision is required in theautomatic gain control routine, the determination of peak amplitudeusing the quadratic interpolation can also be carried out.

Now, the error check routine called cycle skip recovery logic at step 30of FIG. 4 will be described below. In this step, after determining thearrive time Tt by the high threshold detection process, it is determinedwhether it is output as the arrival time or not. This error checkroutine determines whether the high threshold detection process hasproduced the arrival time incorrectly or not. If it has been determinedthat the arrival time has been produced incorrectly, the arrival timewhich has been obtained in the previous detection process is outputagain. Moreover, this error check routine determines whether it isnecessary to override the automatic gain control routine and thedetection time period control routine if the detection process is notbelieved to be correct.

In accordance with a preferred embodiment of the present invention, asdescribed previously, in the case where the noise detected during thenoise detection for the base line of a digital waveform is classifiedinto three kinds of low, intermediate and high noises, if the noise isclassified as the low noise and the detection result by the low and highthreshold detection is consistent, then the detection result isdetermined to be valid by this error check routine. On the other hand,if there is no consistency for the detection result by the low and highthreshold detection or the noise is classified as the intermediate orhigh noise, then this error check routing determines this detectionresult to be invalid. In the above-described embodiment, theinterpolation processing is carried out in the case of the highthreshold detection and no interpolation processing is carried out inthe case of the low threshold detection. This is because, the resolutionof 10 micro seconds is normally sufficient for the low thresholddetection. However, if a higher resolution is required also in the lowthreshold detection, it is of course possible to carry out similarinterpolation processing also in the low threshold detection.

Then, as shown in FIG. 4, upon completion of error check step 30, theentire detection sequence shown in FIG. 4 is repetitively carried outfor the same digital waveform and at minimum the detection sequence isrepeated twice for the same digital waveform. This is because, in thecase where the detection sequence of FIG. 4 is carried out for the firsttime for a specific digital waveform, the detection sequence is carriedout using the values set by the user or the values obtained in theproceeding detection sequence, and in the case of carrying out thedetection sequence for the second time, use may be made of the thresholdset for the peak amplitude which has been obtained by carrying out thedetection sequence for the first time for the specific digital waveform.Thus, although to carry out the entire sequence twice repetitively forthe same digital waveform allows to significantly enhance the accuracyof the result obtained, to carry out the entire sequence three times isnot necessary advantageous because of an increase in calculation time.However, it is true that the more the number of repetitions, the higherthe reliability.

Although the specific modes of embodiment of the present invention havebeen described above, the present invention should not be limited onlyto these specific embodiments and various modifications can, of course,be made without deviating the technical scope of the invention.

Effects

In accordance with the present invention, digital processing is carriedout at downhole and it is only necessary to transmit necessary minimumamount of data to a ground surface processing apparatus by way oftelemetry communication, so that the possibility of occurrence of errorsis minimized. In addition, since the bandwidth of telemetrycommunication is relaxed, the conditions for telemetry communication arerelaxed and a reduction in cost is possible. Besides, since the time fortelemetry communication is minimized, the sonic logging operation isincreased in speed.

What is claimed is:
 1. A sonic logging method for determining characteristics of a formation through which a borehole passes using a downhole tool which is located in the borehole so as to be moveable up and down and coupled to ground surface processing apparatus by means of a logging cable, the downhole tool including: (i) at least one sonic wave generator and at least once receiver spaced apart from each other; and (ii) a downhole processing device operatively coupled to the generator and receiver and also to the ground surface processing apparatus through the logging cable, the method comprising: (a) locating the downhole tool in the borehole; (b) generating a sonic wave signal with the at least one generator; (c) receiving the sonic wave signal with the at least one generator; (d) processing a received signal using the downhole processing device and determining an arrival time for the sonic wave signal at the receiver; and (e) transmitting the determined arrival time to the ground surface processing equipment through the logging cable.
 2. A method as claimed in claim 1, wherein the step of processing the received signal comprises: (i) setting an arrival time detection period; (ii) identifying a point in the arrival time detection period at which the received signal first exceeds a predetermined threshold level; and (iii) determining the point as the arrival time.
 3. A method as claimed in claim 2, wherein the step of processing the received signal comprises: (i) setting an offset detection period earlier than the arrival time detection period; (ii) detecting, during the offset detection period, any DC offset in the received signal baseline; and (iii) determining, from the determined offset, a zero level of the received signal.
 4. A method as claimed in claim 2, wherein the step of processing the received signal comprises: (i) setting a noise detection period earlier than the arrival time detection period; (ii) detecting, during the noise detection period, any noise in the received signal baseline; and (iii) evaluating a reliability for the determined arrival time from the detected noise.
 5. A method as claimed in claim 1, wherein the step of processing the received signal comprises: (i) sampling the received signal at a predetermined sampling interval; (ii) converting the sampling signal into a digital form; (iii) storing the digital form in memory; and (iv) processing the digital form so as to determine the arrival time.
 6. A method as claimed in claim 5, wherein the step of processing the digital form comprises: (i) setting an arrival time detection period; (ii) identifying a point in the arrival time detection period at which the digital form first exceeds a predetermined threshold level; and (iii) determining the point as the arrival time.
 7. A method as claimed in claim 6, wherein the step of processing the digital form comprises: (i) setting an offset detection period earlier than the arrival time detection period; (ii) detecting, during the offset detection period, any DC offset in the digital form baseline; and (iii) determining, form the determined offset, a zero level of the digital form.
 8. A method as claimed in claim 6, wherein the step of processing the digital form comprises: (i) setting a noise detection period earlier than the arrival time detection period; (ii) detecting, during the noise detection period, any noise in the digital form baseline; and (iii) evaluating a reliability for the determined arrival time from the detected noise.
 9. A method as claimed in claim 6, wherein the step of identifying the point at which the digital form exceeds the threshold comprises: (i) identifying a first sample point in the arrival time detection period at which the digital form first exceeds the predetermined threshold; and (ii) interpolating between the first sample point and a sample point immediately preceding the first sample point and identifying a point exceeding the threshold level at a time interval smaller than the sampling interval, which point is determined as the arrival time.
 10. A method as claimed in claim 1, further comprising determining an amplitude for the received signal in the downhole processing device and transmitting this amplitude to the ground surface processing equipment.
 11. A method as claimed in claim 10, wherein the step of processing the received signal comprises: (i) determining a threshold for detecting arrival of a signal; (ii) detecting a point at which the received signal exceeds the threshold; (iii) identifying a largest amplitude in the received signal after the point at which the received signal exceeds the threshold; and (iv) adjusting the threshold to be a predetermined portion of the largest amplitude.
 12. A method as claimed in claim 11, wherein the received signal is subsequently re-processed using the adjusted threshold to determine the arrival time by identifying a point at which the received signal exceeds the adjusted threshold for the first time.
 13. A method as claimed in claim 5, further comprising determining an amplitude for the digital form in the downhole processing device and transmitting this amplitude to the ground surface processing equipment.
 14. A method as claimed in claim 13, wherein the step of processing the received signal comprises: (i) determining a threshold for detecting arrival of a signal; (ii) detecting a point at which the digital form exceeds the threshold; (iii) identifying a largest amplitude in the digital form after the point at which the received signal exceeds the threshold; and (iv) adjusting the threshold to be a predetermined portion of the largest amplitude.
 15. A method as claimed in claim 14, wherein the digital form is subsequently re-processed using the adjusted threshold to determine the arrival time by identifying a point at which the digital form exceeds the adjusted threshold for the first time.
 16. A sonic logging method for determining characteristics of a formation through which a borehole passes using a downhole tool which is located in the borehole so as to be moveable up and down and operatively coupled to ground surface processing apparatus, the downhole tool including: (i) at least one sonic wave generator and at least one receiver spaced apart from each other; and (ii) a downhole processing device operatively coupled to the generator and receiver and in communication with the ground surface processing apparatus, the method comprising: (a) locating the downhole tool in the borehole; (b) generating a sonic wave signal with the at least one generator; (c) receiving the sonic wave signal with the at least one receiver; (d) processing a received signal using the downhole processing device and determining an arrival time for the sonic wave signal at the receiver; and (e) transmitting the determined arrival time to the ground surface processing equipment.
 17. A sonic logging downhole tool for determining characteristics of a formation through which a borehole passes, comprising: (a) at least one sonic wave generator; (b) at least one receiver capable of receiving the sonic wave after it has traveled through the formation; (c) a control device for controlling the generation and reception of the sonic wave, the control device comprising: (i) an analog-to-digital converter for digitizing a detection signal waveform from the at least one receiver at a predetermined sampling interval; (ii) a first memory for storing the digitized waveform; (iii) a second memory for storing a predetermined program for processing the stored digitized waveform; and (iv) a microprocessor capable of executing the program stored in the second memory so as to process the digitized waveform stored in the first memory to determine an arrival time of a sonic wave generated by the generator and received by the receiver.
 18. A sonic logging system for determining characteristics of a formation through which a borehole passes, comprising a ground surface processing apparatus, and a downhole tool in communication with the ground surface processing apparatus and comprising: (a) at least one sonic wave generator; (b) at least one receiver capable of receiving the sonic wave after it has traveled through the formation; (c) a control device for controlling the generation and reception of the sonic wave, the control device comprising: (i) an analog-to-digital converter for digitizing a detection signal waveform from the at least one receiver at a predetermined sampling interval; (ii) a first memory for storing the digitized waveform; (iii) a second memory for storing a predetermined program for processing the stored digitized waveform; and (iv) a microprocessor capable of executing the program stored in the second memory so as to process the digitized waveform stored in the first memory to determine an arrival time of a sonic wave generated by the generator and received by the receiver. 